The objective of the proposed research is an understanding of lipolytic enzyme action on mixed-lipid monolayer systems. Specifically, this research will attempt to evaluate effects of surface charge, molecular spacing of substrate molecules, and surface pressure on enzyme activity. The effects of phase separation and monolayer compressibility as they affect enzyme penetration and action, and the effects of certain surface-active drugs, such as local anesthetics and salicylates on enzyme action will also be investigated. This research will concentrate mainly on a study of two enzymes phospholipase A2 and lipase from various sources. The lipid substrates best suited for these enzymes are short chain (C8-C10) lecithins and glycerides which form insoluble monolayers and whose enzymatic products are readily soluble and thus removed from the interface. Mixed monolayers of these neutral lipids with anionic, cationic or other neutral lipids will be used to study the effects of surface charge and molecular spacing on enzyme action. With mixed-lipid monolayers it is necessary to understand the physical state of the system, particularly with respect to miscibility and phase behavior. Thus, the first objective of this research will be an understanding of the physical characteristics of mixed-lipid monolayer systems. Once these systems are understood, the action of lipolytic enzymes on these monolayers will then be studied.